Process for production of radioactive iron



Patented ct. 16, 1951 PROCESS FOR PRODUCTION OF RADIOACTIVE IRON John A.Swartout, Oak Ridge, Tenn., assignor to the United States of America asrepresented by the United States Atomic Energy Commission No Drawing.Application September 7, 1950, Serial No. 183,657

8 Claims. 1

My invention relates to the production of radioactive iron, andparticularly to an improved process for the production of radioactiveiron of relatively high specific activity, which is adapted for usewhere only a low or moderate neutron flux is available.

Radioactive iron of high specific activity can be prepared by neutronbombardment of iron or an iron compound, for a suiiicient time toapproach saturation equilibrium, if a neutron flux of about 10 neutronsper sq. cm. per second, or higher, is available. In view of theextremely small number of sources of neutron flux of this magnitudethere has been a need for a suitable process for the production ofradioactive iron of relatively high specific activity utilizing fluxessubstantially below 10 neutrons per sq. cm. per second.

When iron or an iron compound is bombarded at a low or moderate neutronflux, the specific activity at saturation equilibrium is undesirablylow, and attempts have therefore been made to secure a concentration orenrichment of the radioactive product. Such attempts have generallyutilized the Szilard-Chalmers efiect, in accordance with which a,portion of the bombarded material is found to differ in chemicalcomposition from the original material, and this portion, together withits associated radioactive product, may then be separated by chemicalmeans, thus resulting in a concentration Or enrichment. TheSzilard-Chalmers reactions thus far employed, however, have generally'not produced products of as high specific activity as desired, and haverequired adsorption, extraction, carrier precipitation, or other complexseparation procedures.

An object of my present invention, therefore, is to provide an improvedprocess for the production of radioactive iron of relatively highspecific activity which requires the use of only a low or moderateneutron flux.

Another object of my invention is to provide such a process whichemploys an extremely simple separation procedure for efiecting thedesired concentration or enrichment.

' A further object of my invention is to provide such a process which isreadily adaptable to continuous operation.

Additional objects and advantages of my invention will be evident fromthe following description.

In accordance with my present invention magnesium ferrocyanide issubjected to neutron bombardment and the water-insoluble fraction ofthe-bombarded material, together with its associated radioactive iron,is separated from the water-soluble fraction. The separation procedureof my process thus involves only the very simple expedient of separatina precipitate from an aqueous solution. Utilizing controlled bombardmentconditions, as hereinafter described, this process is capable ofconcentrating the bulk of the radioactive iron in a very small insolublefraction of the bombarded material, thus effecting a high degree ofconcentration or enrichment.

The magnesium ferrocyanide to be bombarded in accordance with thepresent invention may be in any convenient form, but is preferably purecrystalline Mg2Fe(CN)s-12H2O. Aqueous solutions may also be employed, aswill later be discussed, but my invention will be particularly describedand illustrated with reference to the use of the crystalline salt. Formost purposes the magnesium ferrocyanide may contain iron of naturalisotopic distribution, which will result in a product containing Fe andFe in a ratio of about 1.25 millicurie of Fe per millicurie of Fe. TheFe in such a product is of. most interest in view of its half-life of 46days and its relatively much harder radiation. The four year half-lifeFe in a neutron bombarded compound of natural iron can be neglected inmost chemical tracer uses, but is undesirable for many biological uses.If the product is to be employed for biological purposes, therefore, itis desirable to prepare the magnesium ferrocyanide from iron enriched inFe or from the pure isotope Fe. For special purposes other enrichmentsor other pure isotopic compounds can, of course, also be employed.

It is desirable to remove any water-insoluble constituents from themagnesium ferrocyanide prior to bombardment, especially if acommercially available salt is to be used. For this purpose it issufiicient to dissolve the salt in water, to about 0.1 M concentration,remove any insoluble matter by centrifugation or other suitable means,concentrate the remaining solution, and recrystallize the salt. 7Themagnesium ferrocyanide, purified as described above, or b anyequivalent procedure, is then placed in a suitable container for neutronbombardment. Thin-walled quartz vessels are desirable for this purpose,and such vessels can suitably be protected from mechanical shock and thelike by means of outer containers of aluminum, zirconium, or othermaterial of low neutron absorption cross-section. f

Any of the known types of neutron sources may be employed for bombardingthe magnesium ferrocyanide in accordance with my invention.

Although cyclotrn bombardment of certain target materials can yieldneutron fluxes above n/sq. cm./sec; the preferred neutron sources areuranium or plutonium nuclear reactors, particularly reactors providing athermal neutron flux ofabout 10 n/sq. cm./sec. to 10 n/sq. cm./sec. Ifthe nu.- clear reactor operates at a neutron energy substantially abovethe thermal range, or if another type of neutron source is employedwhich provides high energy neutrons, it is preferred to interpose amoderator such as carbon, beryllium, or a hydrogenous material, betweenthe source and the magnesium ferrocyanide being, bombarded.

It should be understood. that. my processis completely operativewhenemployinga neutron flux of 10 n/sq. cm./sec..or higher itsadvantages over other available processes. are. merely less pronouncedat a very hignneutron flux.

The preferred time of bombardment or the.

magnesium 'ferrocyanide. .will. depend." on the.

neutron flux employed, but. can be. expressed in.

terms of. the saturation time, 1.. ethe time. beyond which. further.production. of. radioactive. product is. offset byradioactive. decay,resulting in. an equilibrium concentration or specific. ac.- tivity.The. bombardment time. should be lessthan 0.5 andpreferably less, than0.l.of.the,-sat.uration. time. With. neutron. fluxesE in. the range. 10-10 n/sq. cm./s.ec., bombardment times. of 1-50 daysmay suitably beemployed, with. atime. of. less than 15 days being preferred.

At neutron fluxes below about. 10 n/sq. cm./sec. the specific activityof the-productobtained', by the present processdecreases with .in.--

creasing timeof' bombardment even though. the total quantity of activeproduct increases. with. increasing time of bombardment... If maximum.specific activity is desired, therefore, the time. of. bombardmentshould be only long enough to. produce the minimum separable insoluble.fraction. in the bombardment product. This minimum, about 10- gram ofiron. in. the insoluble. fractionper. gram of Mg2Fe(CN)s-1-2I-IzQbombarded, is attainable with abomhardment time of about. 1 2hrs. at a neutron. flux of. about. 10.1 n/sq. cm./sec. Somewhat longer.bombardments, giving an insoluble fraction containing. about 10.-

gram. of. iron pergram of. MgzFe-(CNJalZHzQ bombarded, are generallydesirable: to facilitate. the. separation of the insoluble material-The. magnesium. ferrocyanide may be born..- barded at any temperaturebelow its thermal de-- composition temperature, i. e., about'. 200 0..

ill

However, even minute amounts of thermal decomposition. products tendtodilute the. insoluble fraction of the bombarded material and thusreduce its specific activity. It is generally de-- sirable, therefore,to eifect the bombardment ata temperature below 150 C., and it ispreferred-,.

especially for bombardments of relatively short.

duration, to maintain the magnesium ferro: cyanide within the range -90C., during bombardm'ent.

At the conclusion of the bombardment the container of bombarded.material is removed. from the neutron flux and preferably. storedina.

shielded area for 2 hrs. or longer to permit radioactive decay of the Mg(10 min. half-life) and v of any short-lived activities induced in thecon- 0.05-0.25 M; and preferably about 0.1 M with.

respect to MgFe CN s 121-120.

The resulting insoluble precipitate is then separated from thesupernatant solution by any suitable procedure such as sedimentation orfiltration, but preferably by centrifugation. This precipitate; whichwill generally contain about 8%, of the total. radioactivity of thebombarded material, may be merely dried for use as such, or may beignited or subjected to other treatment to produce-a product of thedesired chemical constitution for its proposed radioactive application.The magnesium ferrocyanide in the separated supernatant solution may, ofcourse, be recrystallized for further bombardment. 1

It willbeevident that the process previously described is readilyadaptable to continuous op.- eration by the. expedient of bombardingthe. mag.- nesium ferrocyanide in the solution from which. the insolublebombardment product is to be 'sepae rated. For this purpose H2O or D20solutions. of magnesium ferrocyanide may be continuously cir-- culatedthrough anuclear reactor, the insoluble. bombardment product may becontinuouslyseparated by'centrifugation, and the supernatantso-. lutioncontinuously recycled to. the reactor. 7

Neutron. fluxes, temperatures, bombardment. times, and concentrationswhich are. suitable for. the crystal bombardment process may. also. be.employedforthe, continuous solution bombard ment process. Temperatures.substantially below. 100" C. are preferred, however, in order tolavoid"the use of superatmospheric pressure. I The restdence time. in thenuclear reactor; per cycle, should also preferably be somewhat shorter:than the corresponding. bombardment time for the. crystalline material,with a circulation rate sum;- ciently high to preventdeposition of.theinsoluble. bombardment product in partsof the apparatus other thanthecentrifuge.

My invention will now befurther illustratedb'y means of specificexamples. involving the loom bardment of crystalline MgzFe(CN)e-1'2H2.0"of. natural isotopic contentz.

Example I Recrystallized, MgzFe(CN)e-l2HzO. was placed in afuzed quartvesselwithin: an. aluminum. outer container, which. was positionediniawaaterjacketed tube within. a nuclear. reactor. The

water flowing through the jacket of. the tubawas maintained at C., andthe flux. level in the M. with respect to Mg2Fe(CN).e-12H2O, and .the

resulting precipitate was separated-by centrifuggation.. The ironcontent of the precipitateswas determined by theorthophenanthroline-ferrous, complex colorimetric method, afterdissolving: in- HCl- The Fe activities of aliquots of the precipie tate,the supernatant solution, and the unsepa--- rated. irradiated salt werethenrdeterminedmeans of conventional counting methods, em-

ploying a high-pressure argon-filled ionization Example II Portions ofMgzFe(CN)s-12H2O were bombarded as in Example I, for various times asshown in the table below. The bombarded materials'were then treated andanalyzed as in Example I, with the additional determination of the ironcontent of the precipitate per gram of original Mg2Fe(CN)s-12H2O. Thisvalue, together with the specific activity of the bombarded salt, and ofthe precipitate, are shown in the table below:

W f F Specific Activity (ll)ll1lit. e in curies per gram ggfigi' gPrecipitate For (days) g. of Original salt gfi Precipitate 3. 75 l.22X10- 0. O5 35. 6 5. 2 1 63X10- 0. 06 34. 7 6. 6 2 23X10 0.08 32. 8 l2.3 4 78X10- 0. 31. l 79.0 27 5 Xl0- 0.63 23.2

iron which comprises subjecting crystalline Mg2F'e(CN)s-12H2O to neutronbombardment at a flux of 10-10 n/sq. cm./sec. for a time sulficient toform a separable water-insoluble fracv tion of the bombarded materialbut less than 0.5 of the sautration time, introducing the bombardedmaterial into water to dissolve the unchanged magnesium ferrocyanide,and separating the resulting precipitate from the supernatant solution.

3. A process for the production of radioactive iron which comprisessubjecting crystalline Mg2Fe(CN)s-12HzO to neutron bombardment at a fluxof 10 -10 n/sq. cm./sec. for a time sufiicient to form a separablewater-insoluble fraction of the bombarded material but less than 0.1 ofthe saturation time, while maintaining the temperature of saidMg2Fe(CN)5'12H2O below 150 0., introducing the bombarded material intowater to form a 0.05-0.25 M solution of the unchanged magnesiumferrocyanide, and separat- .zsn 2. A process for the production ofradioactive ing the resulting precipitate from the supernatant solution.

4. A process for the production of radioactive iron which comprisessubjecting crystalline Mg2Fe(CN)6-12H2O to neutron bombardment at a fluxof 10 -10 n/sq. cm./sec. for a time suflicient to form a separableWater-insoluble fraction of the bombarded material but less than 0.1 ofthe saturation time, while maintaining the temperature of saidMg2Fe(CN)s-12H2O within the range 20-90 C., introducing the bombardedmaterial into water to form an approximately 0.1 M solution of theunchanged magnesium ferro cyanide, and separating the resultingprecipitate from the supernatant solution.

5. A process for the production of radioactive iron which comprisessubjecting crystalline Mg2Fe(CN)s-12H2O to neutron bombardment at a fluxof approximately 10 n/sq. cm./sec. for 0.5 to 15 days while maintainingthe temperature of said Mg2Fe(CN)s-12H2O within the range of 20-90 C.,introducing the bombarded material into water to form an approximately0.1 M solution of the unchanged magnesium ferrocyanide, and separatingthe resulting precipitate from the supernatant solution.

6. A process for the production of radioactive iron which comprisescontinuously circulating an aqueous solution of magnesium ferrocyanidethrough a zone of neutron flux, continuously withdrawing said solutionfrom said zone of neutron fiux, continuously separating insolublematerial from said withdrawn solution, and continuously recycling theresulting supernatant solution to said zone of neutron flux.

7. A process for the production of radioactive iron which comprisescontinuously circulating a 0.05-0.25 M solution of magnesiumferrocyanide through a zone of neutral flux of 10 -10 n./sq. cm./sec.while maintaining the temperature of said solution within the range20-90 C., continuously withdrawing said solution from said zone ofneutron flux, continuously separating insoluble material from saidwithdrawn solution, and continuously recycling the resulting supernatantsolution to said zone of neutron fiux.

8. A process for the production of radioactive iron which comprisescontinuously circulating an approximately 0.1 M solution of magnesiumferrocyanide through a nuclear reactor having a neutron flux of about 10n./sq. cm./sec. while maintaining the temperature of said solutionwithin the range 20-90 C., continuously withdrawing said solution fromsaid nuclear reactor, continuously separating insoluble material fromsaid withdrawn solution, and continuously recycling the resultingsupernatant solution to said nuclear reactor, the average residence timeof the solution in said nuclear reactor, per cycle, being of the orderof 1 day.

JOHN A. SWARTOUT.

REFERENCES CITED The following references are of record in the file ofthis patent:

Thompson, Some Exchange Experiments Involving Ferrocyanide andFerricyanide Ions, U. S. Atomic Energy Commission, declassified documentMDDC-790, March 9, 1947, 4 pages.

Williams, Journal of Physical and Colloid Chemistry, volume 52, pages603-611 (1948).

1. A PROCESS FOR THE PRODUCTION OF RADIOACTIVE IRON WHICH COMPRISESSUBJECTING MAGNESIUM FERROCYANIDE TO NEUTRON BOMBARDMENT AND SEPARATINGTHE RESULTING WATER-INSOLUBLE FRACTION FROM THE BOMBARDED MATERIAL.